Method and device for catalytic oxidation and reduction of gases and vapours by crystalline compounds of heavy metals and rare earths

ABSTRACT

The invention describes the production of a catalyst for use in the oxidation and selective reduction with properties resembling the catalytic property of the noble metal catalysts, by formation of a crystal surface on catalyst supports from a synthesis crystal of the elements of the rare earths and the metal components cobalt and/or lanthanum produced in multiple stages.

The invention describes the method for producing such catalysts and the use in catalytic reaction systems as a substitute for platinum catalysts and redox systems such as for example the DeNO_(x) synthesis.

Such catalysts are known as honeycomb or pouring layer catalysts from DE 198 00 420 A1 and U.S. Pat. No. 4,707,341, respectively. Therein, aqueous solutions from a first component of a compound with lanthanum, cerium and cobalt, a second component platinum, a third component rhodium as well as a fourth component from a wash coat of alumina, titanium dioxide and oxalic acid are applied to the catalyst.

The disadvantage of these catalysts is the small surface of the active substance, which is supplemented to a sufficient catalytic effect by the additional use of platinum and rhodium. Thereby, the compounds of lanthanum, cerium and cobalt only act as an improved surface for the platinum, and allow an increased lifetime. Surprisingly, now, it has been found that the level of platinum is almost completely substituted, the lifetime is increased and the catalytic effect is substantially improved, if the active substance is crystallized from rare earths and cobalt in multiple stages. Therein, also the substances cobalt and manganese and the rare earths lanthanum, cerium and yttrium are used, but transformed to crystals with a diameter from 1 to 0.1 μm and a length from 100 to 100,000 μm in a special multi-stage process.

According to the inventive method, this occurs in the following steps:

The salts of the rare earths lanthanum, cerium and/or yttrium and the salts of the metals cobalt and/or manganese are input into deionised water in the stoichiometric ratio until a 20 to 60% solution arises. Therein, the stoichiometric ratio is the amount corresponding to the atomic weight of the involved masses of the reaction partners to the complexes LaCoO₃, wherein La can be replaced by cerium and yttrium and cobalt can be replaced by manganese. As the salts, the nitrates, carbonates and acetates are used.

The subsequent burning operation requires a temperature between 500 and 761° C. The maximum temperature is preset since this temperature is the start of another recrystallization that has to be avoided. The coarse crystal mixture arising in the heating operation is again dissolved in 20 to 60% oxalic acid until the bottom is low. It is removed from the solution by redecantation. The pure bottom-free solution is again heated to the burning temperature of 500 to 600° C. and thus processed to a fine powder in crystal form.

Now, another oxalic acid solution is admixed in water with 20 to 40% oxalic acid. Molecularly fine alumina (Condea), 10%, and fine Bayer titanium or equivalent TiO₂ is now mixed into this solution. The crystal powder produced above is now lastly input into the solution. The thus resulting mixture is intensively stirred to a thin-liquid suspension, possibly with addition of further water.

With this suspension, the actual catalyst production is effected by submerging the support bodies having a honeycomb or pouring layer structure into this suspension. Thereby, it is necessary that the bodies be completely submerged, since else the fusion effects distribute the catalyst unevenly on the surface.

The thus produced wet catalysts are processed to finished catalysts in a “calcination furnace” at 450 to 550° C. over at least 12 hours. For improved “initiation” of the catalytic reaction, after the burning process, the catalyst is submerged in a platinum or palladium nitrate solution, such that a concentration of platinum or palladium from 0.1 to 0.5 g/l of catalyst results. Also, the thus resulting finished catalyst is calcined once again over 6 hours at 450 to 550° C.

The resulting catalyst bodies have a uniform crystal structure or crystal layer, respectively, on the surface, which can be seen in a microscope or scanning electron microscope. Therein, the concentrations determinable in the scanning electron microscope (SEM) between the rare earths and cobalt or manganese, respectively, are not allowed to fall below the stoichiometric ratio at any point. The rare earths always have to be present at least stoichiometrically or super-stoichiometrically. Also, the distribution of the platinum or palladium, respectively, has to be on the crystals in at least 30%, and must not be exclusively bound to the Condea or the titanium dioxide. This is analyzed by the REM with the laser analysis of the crystal surface.

In a special embodiment, the invention is explained in more detail. The embodiment shows the production of the crystal catalysts according to the invention, i.e. catalysts with applied crystal layers with platinum-like properties by way of the starting materials of the rare earths, lanthanum and cerium and the heavy metal reaction partner cobalt. The aim of the embodiment is to produce the complex La_(0.9)Ce_(0.1)CoO₃ such that no free cobalt is available for decomposition of the crystal, and the complex is completely neutral and non-toxic.

For this, the components lanthanum acetate, La(CH₃COO)₃×n H₂O, with a content of La₂O₃ of 40%, cerium acetate, Ce(CH₃COO)₃×n H₂O with a cerium content of CeO₂ of 45%, and cobalt acetate Co(CH₃COO)₂×4 H₂O with a content of cobalt of 24%, are mixed in the following manner:

430 g lanthanum acetate+45 g cerium acetate+250 g cobalt acetate are mixed into a bath with 3 litres of deionised water in dissolved manner. The solution is brought to temperatures in the vicinity of 100° C. while stirring, and subsequently heated to 600° C. without stirring. Therein, the substances react to a black powder containing the complex La_(0.9)Ce_(0.1)CoO₃ in 90% and a mixture of La_(0.9)Ce_(0.1) in 10%.

This complex is stable by the excess of La_(0.9)Ce_(0.1) and prevents excessive cobalt from being able to become active both with respect to the heavy metal loading and with respect to the thermal decomposition of the complex. Thus, the excess of complexes of the rare earths is an important feature of the resulting catalyst crystal. 300 g black catalyst powder result.

This catalyst powder is again dissolved in a solution of 300 g oxalic acid and 3 litres of water while heating and again heated to 500° C., wherein a similar black powder arises, but which has a much finer crystal structure and a much more uniform element distribution under the scanning electron microscope. In the case of addition of other rare earths and manganese, the process is repeated a third time until the complete homogeneity is achieved.

The thus resulting black catalyst powder of 300 g needle crystal material, also called perovskite, is now again stirred in 4 litres of deionised water with 105 g oxalic acid, 100 g Condea and 50 g Bayer titanium, a microgranular titanium oxide. The ceramic bodies to be steeped, which are to be conditioned to catalysts by submerging in this solution, are previously dried.

As the catalyst bodies, a honeycomb body, wound metal bodies with continuous channels and porous ceramic extrudates are suitable. As the ceramic material for these bodies, the materials magnesium-aluminium-silicate, the cordierite, SiO₂ bodies, titanium-tungsten-oxide-honeycombs and alumina have turned out, wherein the materials magnesium-aluminium-silicate, cordierite, SiO₂ bodies, titanium-tungsten-oxide honeycombs are particularly insensitive to expansion and thus particularly suitable.

The catalyst supports are submerged in the catalytic solution such that the porous bodies are uniformly coated, that is, they have to be completely and quickly submerged in order to permit capillary liquid transport within the ceramic as little as possible. If the body is submerged too slowly or on one side, the capillary force of the ceramic body soaks the liquid and the catalytic substance is filtered such that the coating is effected non-uniformly.

After coating by submerging, excessive liquid is separated. This is effected in that the body is placed on a base covered with a screen, which absorbs the excessive liquid. After complete settling of the liquid at the lower end of the honeycomb bodies, they are freed from the liquid at the lower end by shaking-off or slight blow-out such that the bores of the honeycomb bodies are free.

After coating, the activation of the catalytic coating is effected in that these bodies are calcined at a temperature of 500° over a time period of 2 to 20 hours, that is, the oxalic acid is then completely burned out of the honeycomb body. The burning time depends on the size of the honeycomb bodies. The larger the honey comb body, the longer the burning time.

The honeycomb body thus provided with a crystal layer is not yet completed for the catalytic efficiency. According to the invention, it has been found that small amounts of noble metals, which are applied thereafter in a separate coating operation, develop a particular efficiency. This is because the noble metals are preliminarily deposited on the crystals of the La_(0.9)Ce_(0.1)CoO₃ and La_(0.9)Ce_(0.1). This is effected by steeping with a noble metal solution, such as for example platinum nitrate, in such a concentration that 1 gram platinum is dissolved in 1 litre deionised water, and by subsequently burning the ceramic bodies also at 500° C.

Thereby, with respect to the usual noble metal coatings, a very stable positioning of the noble metals results, which achieve a much higher lifetime than upon coating with Condea and Bayer titanium. Moreover, the noble metal brings the coating of La_(0.9)Ce_(0.1)CoO₃ and La_(0.9)Ce_(0.1) as an “ignition metal” faster to a full catalytic activity, that is, the catalytic activity of the platinum or other noble metals “ignites” the catalytic activity of the lanthanum-cerium-cobaltite. This interaction also results with catalyst toxins, which differently poison the two catalytic systems La_(0.9)Ce_(0.1)CoO₃ and noble metals. The respectively less poisoned system activates the other system.

As a result of the inventive coating, the following advantages result, which also establish the economy of the system. The comparable noble metal coating of the same activity necessitates a 20 times higher noble metal amount, has only ca. 5% % of the lifetime compared to the inventive coating, and the catalyst toxins have a reduced influence on the inventive coating.

Moreover, besides the catalytic oxidation of hydrocarbons, the inventive coating gives a completely new, surprising effect. These honeycomb bodies are capable of selectively removing nitrogen oxides from exhaust gases, that is, the nitrogen oxides are also reduced with oxygen containing exhaust gases and the remaining oxygen does not react with the lanthanum-cerium-cobalt surface. However, the process proceeds only as long as until the highest oxidation stage of the catalyst is achieved. Thereafter, the catalyst has to be regenerated again, which is possible with CO and H₂ according to today's prior art. 

1. Method for producing catalysts for oxidation of gaseous and vaporous hydrocarbons (VOC) and catalytic, selective reduction of DeNO_(x), characterized in that with compounds of rare earths and heavy metals cobalt and manganese, a crystal layer is formed on support bodies as a catalytically active substance in a multi-stage crystallization process.
 2. Method according to claim 1, characterized in that the compounds of rare earths and heavy metals, cobalt and manganese are formed in super-stoichiometric ratio such that an excess of rare earths results in the crystal layer.
 3. Method according to claim 1, characterized in that the rare earths are lanthanum, cerium and yttrium and are used collectively or singly.
 4. Method according to claim 1, characterized in that as the heavy metals, the metals cobalt and manganese are used.
 5. Method according to claim 1, characterized in that starting substances include salts, which are dissolved in water and oxalic acid.
 6. Method according to claim 1, characterized in that formation of the complex of rare earths and heavy metals is effected by heating the mixture to 500 to 761° C., and this operation is repeated at least once by dissolving in oxalic acid water.
 7. Method according to claim 1, characterized in that the catalytically active substance is stirred in an oxalic acid mixture with addition of molecular alumina, Condea and titanium oxide, Bayer titanium to a seeping solution.
 8. Method according to claim 1, characterized in that a finished coated catalyst is activated by heating to 500° C.
 9. Method according to claim 1 characterized in that a finished catalyst is once again seeped in a noble metal solution and subsequently heated to 500° C. such that the noble metal concentration on the body is between 0.05 and 0.5 g/l. 